The venting of hydrocarbon vapors into the atmosphere has been a common practice at many petroleum production and processing facilities. Often, where the amounts are substantial, the vapors are collected, recompressed and used. There are many other locations where these vapors are vented to the atmosphere or flared. Recently, as of 2012/2013, the United States Environmental Protection Agency (EPA) has placed an upper limit on the amount of volatile organic vapors (VOCs) that may be vented. There is also a desire to minimize the venting of methane gases or gases that decompose to methane to the atmosphere because methane (CH4) has a strong greenhouse gas (heat-trapping) effect, being twenty-one times more effective than carbon dioxide over a 100-year period.
A conventional method of dealing with the vented gases is to combust the gases. The standard method of combusting these gases is to feed these gases to an incinerator unit or flare where a pilot, either continuous or started on demand, feeds into the vented gases in the presence of air to ignite the gases. In the case of a flare, the vent gases are directed through a vertical tube or pipe and burned as the gases contact air. Since a flare is undesirable from an environmental and public perception point of view, the general preference is to enclose the flame and to regulate the air flow to achieve combustion with good air-fuel ratio control. The disadvantage with flares and incinerators is that the heat energy from the vapor combustion is lost and not used. In addition, adding a flare or incinerator to a site may require additional effort to obtain permission for installation and operation by regulatory authorities.
Another approach is to combust the vented gases in a natural gas engine. U.S. Pat. No. 8,382,469 (MaIm) discloses a method and apparatus for utilizing fugitive gases (i.e. vent gases) as a supplementary fuel source for a natural gas engine. This technique provides a number of advantages: fuel savings, elimination of methane, elimination of volatile organic compounds (VOC) and elimination of benzene, toluene, ethylbenzene, and xylene-volatile aromatic compounds (BTEX).
U.S. Pat. No. 8,113,181 (MaIm) discloses a control system for regulating vent gas flow to a natural gas engine. Because a vent source can have an unpredictable and variable flow, the control system provides methods either for limiting the maximum flow to the engine or adjusting the engine air to fuel ratio according to the vent gas flow rate. However, a control system with flow measurement devices that provide signals to a microcontroller, though useful in some circumstances, adds cost and complexity.
Accordingly, a simpler system and method for controlling the flow of vent gases to an engine such as a natural gas engine remains highly desirable.